Work treating with electron beam

ABSTRACT

The electron beam B is projected on work W, usually in the atmosphere outside of the chamber where the beam is generated, through a plurality of aperture members 11, 13, 15, 17. The holes in the aperture members are as small as practicable to suppress the feedback of air into the chamber. The beam B is focussed in regions 85 and 87 (FIG. 3) with reference to the aperture members so as to preclude damage to the members by impingement of the beam on the walls of the holes through which the beam passes. The beam current is varied in dependence on the demands of the work but the focus of the beam is maintained by bias resistor 115 (FIG. 1), without damage to the aperture member, by instantaneous change in the bias impressed on the beam by a control electrode G.

Home States em [72] Inventors Joseph Lempert Pittsburgh; Jerald F. Lowry, Murrysville, both of Pa.; Frederick M. Bonner, Washington, D.C. [21] Appl. No. 30,605 [22] Filed Apr. 21, 1970 [45] Patented Jan. 11, 1972 [73] Assignee Westinghouse Electric Corporation Pittsburgh, Pa. Continuation of application Ser. No. 636,181, May 4, 1967 now abandoned. This application Apr. 21, 1970, Ser. No. 30,605

[54] WORK TREATING WITH ELECTRON BEAM 16 Claims, 5 Drawing Figs.

[52] U.S.Cl 219/121 EB, 250/495, 315/31 [51] Int. Cl B23k 9/00 [50] Field of Search 219/69, 117,125 PL, 121 EB, l21;3l5/31; 250/495 [56] References Cited UNITED STATES PATENTS 2,902,583 9/1959 Steigerwald 219/50 Primary Examiner-.1. V. Truhe Assistant ExaminerGale R. Peterson AttorneysA. T Stratton and Z. L. Dermer ABSTRACT: The electron beam 13 is projected on work W, usually in the atmosphere outside of the chamber where the beam is generated, through a plurality of aperture members 11, 13, 15, 17. The holes in the aperture members are as small as practicable to suppress the feedback of air into the chamber. The beam B is focussed in regions 85 and 87 (FIG. 3) with reference to the aperture members so as to preclude damage to the members by impingement of the beam on the walls of the holes through which the beam passes. The beam current is varied in dependence on the demands of the work but the focus of the beam is maintained by bias resistor 1 15 (FIG. 1), without damage to the aperture member, by instantaneous change in the bias impressed on the beam by a control electrode G.

GENERATOR PATENIEB mu 1 I972 SHEET 1 [IF 3 POWER MAINS RADIAL DISTANCE FROM AXIS (INCHES) IATENTED JAN] 1 I972 sum 2 [IF 3 FIG. 3

EFFECTIVE CENTER I7 75 I I I EFFECTIVE CENTER FIG.4

CONTROL ELECTRODE POTENTIAL 150 v ANODE POTENTIAL I50 kV I498 kV AXIAL DISTANCE ALONG COLUMN (INCHES) PATENTED JAN] 1 I972 SHEET 3 OF 3 0 0 lllll mu ll||l| m O| Ill x I. f. r I //II III 2 r l. i I I x I H V m n s O 4. B 3 V W m 8 m on mdl WORK TREATING WITH ELECTRON BEAM This application is a continuation of copending application Ser. No. 636,181 filed May 4, 1967, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the utilization of electron beams and has particular relationship to the treatment of work, typically the welding of work, with an electron beam. Specifically this invention concerns itself with work treatment in which an electron beam is generated in a chamber or column and is projected into treating relationship with work located in a region at a higher pressure than the chamber, particularly in the atmosphere outside of the chamber. In such work treatment the electron beam apparatus has aperture means which includes a plurality of aperture members or orifice members extending between regions to which separate exhaust pumps are connected. Throughout the column through which the beam passes pumping takes place at each end of each aperture member except the last aperture member through which the beam passes to the work. The ends of each aperture members including the last one are at different pressures. Such a pumped aperture system is described as having differentially pumped aperture member.

The column through which the beam passes has a plurality of stages at different pressures and it is essential that the flow of gas from higher-pressure to lower-pressure stages be minimized. The holes in the aperture member through which the electron beam passes are then as of small area as practicable; typically less than 0.060-inch diameter.

In treating work it is necessary that the beam properties be set to meet the demands of the work. The electron beam apparatus is provided with facilities for changing the electrical parameters governing the beam properties, particularly its current and voltage, as the work may demand.

In accordance with the teachings of the prior art the beam is aligned and focused between the aperture member and is focused on or near the work. The focusing is effected electrostatically by impressing an appropriate potential on a control electrode disposed to impress an electric field on the source of electrons and electromagnetically by a coil disposed to impress a magnetic field on the beam.

Attempts have been made in the prior art practice to vary the electron-beam current. It was discovered that as the beam current is increased to moderate or high magnitudes, say about 50 to 100 milliamperes, the aperture members are severely corroded and burned. The damage occurs even in tungsten aperture members. It is an object of this invention to overcome the deficiencies of the prior art and to preclude or suppress the damaging of the aperture member of the electron beam apparatus in the treatment of work with variable elec tron beam currents and voltages. It is also an object of this invention to provide a method of, and apparatus for, treating work, with an electron beam which is projected, through differentially pumped aperture members. out of the chamber in which it is generated to a higher pressure region, in the practice and use of which, at different beam powers, the apertures shall not be damaged.

SUMMARY OF THE INVENTION This invention arises from the discovery that the aperture members are damaged in prior art practice because of the shifting of the focal points of the beam with changes in beam power. For an initial beam current the beam is focused, in accordance with the teachings of the prior art, to desired beam crossover positions. As the beam current is changed the crossover regions are shifted. It has been realized that if it happened that the beam does not originally impinge on the walls or the aperture members, such impingement occurs as the current changes and at moderate or high currents the aperture members are damaged.

In accordance with this invention the damage to the aperture members is precluded by maintaining the beam focused, for different values of beam current, at crossover regions so that the orifice or aperture walls are substantially not impinged, or minimally impinged, by the beam throughout the beam current range. With the beam parameters set at predetermined magnitudes, the beam is focused so as not to damage the aperture member. As the parameters are changed the beam is refocused so as not to damage the aperture members. The refocusing is effected so quickly that it is completed before the beam current has impinged on the walls of the aperture members long enough to damage them. Specifically the refocusing is effected substantially instantaneously by automatically changing the grid potential, or grid bias, on the control electrode, or grid, of the beam generator, responsive to changes in the beam current.

The extent to which the focusing is effective may be determined by comparing the current carried by the beam through the aperture members to the work with the total current emitted from the electron source. For most effective focusing the current transmitted to the work should be at or near I00 percent of the current emitted by the source.

To protect against imperfections in maintaining the focusing the focus should be set so that the beam passes through the orifice members with a minimum impingement on their walls at or near the parameters corresponding to the highest beam current setting which would tend to inflict the most serious damage on the apertures. The focus may then be maintained as disclosed in this application as the beam current is decreased.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a view partly schematic and partly diagrammatic showing electron beam apparatus in accordance with this invention used in the practice of this invention;

FIG. 2 is a diagrammatic view which serves, together with table A to be presented herein, to give the dimensional parameters of the cathode-control-electrode-aperture structure of actual apparatus according to this invention used in the practice of this invention;

FIG. 3 is a diagrammatic view showing the manner in which the electron beam is focused in the practice of this invention, the divergence of the beam being shown highly enlarged for the purposes of clarifying the focusing;

FIG. 4 is a graph showing a computed equipotential configuration for assumed potential magnitudes impressed between the electrodes of apparatus according to this invention; and

FIG. 5 is a graph showing computed trajectories at various electron energies for the equipotential configuration shown in FIG. 4.

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 shows electron beam apparatus for welding work W located outside of the evacuated column in which a beam B is generated. This apparatus includes an Electron Beam Generator and a Power Supply. The Generator includes a cathode K, which serves as a source of electrons, a control electrode or grid G, an anode A and a plurality of aperture members 11, 13, 15, 17. Typically the cathode K and electrode G are combined into an assembly which is suspended from an insulator 21, through which the conductors 23, 25, 27, 29 from the Power Supply are sealed.

The cathode K includes a rod or bolt 31 of tungsten or other electron-emissive material. The rod 31 is heated by bombardment with electrons from an electron-emissive filament 33 under a potential impressed between the rod 31 and the filament 33 from conductors 23 and 25. The rod 31 and filament 33 is enclosed in a shell 37 to which one end of the filament 33 is connected and through which heating current is supplied to the filament 33 from conductors 25 and 27.

The control electrode G is a generally cylindrical shell having an internal projection 41 and an outer lip 43 defining coaxia! holes through which the electrons from the rod 31 pass. The anode A is a generally circular plate having an opening through which the electrons pass. A high potential is impressed by conductor 23 between the rod 31 and the anode A, which is grounded.

The cathode K. control electrode G and anode A are in an enclosure 47 through whose wall 49 opposite the cathode K the aperture of member 11 extends and to which it is sealed. This enclosure 47 is evacuated by a diffusion pump (not shown) to a low pressure of the order of less than one-half micron. The apertured member 11 has a tapered opening 51 which typically may have a diameter of about 0.059 inch at the smaller end remote from the cathode K and a taper of about 3 or 4.

The apertured member 13 is a long generally cylindrical rod shouldered at the ends and having a tapered opening 53 having a diameter of about 0.059 inch at the smaller end towards the cathode K and a taper of about 3 or 4. The members 11 and 13 are sealed vacuumtight into an enclosure 55 with their smaller ended openings face to face. The enclosure 55 is maintained evacuated to a pressure higher than the pressure in enclosure 47 by a small pump (not shown). There is a pressure across opening 51 which tends to cause gas to flow from enclosure 55 into enclosure 47 but the flow of gas is minimized by the opening 51 because it is small.

The apertured member has tapered opening 61 with the smaller end, away from the cathode K, typically having a diameter of about 0.051 inch and the taper about 3 or 4. The members 13 and 15 are sealed vacuumtight into an enclosure 63 with their larger diameter ends face to face. The enclosure 63 is pumped by high-power pumps (not shown); the pressure in enclosure 63 is higher than the pressure in enclosure 55 so that there is a differential pressure across member 13 but the small opening in the hole 53 minimizes backflow ofgas.

The apertured member 17 is a generally circular disc with a hole 71 typically of 0.059-inch diameter in the center. The members 15 and 17 are sealed vacuumtight into enclosure 73 which is evacuated by high-power pumps (not shown) but is at a pressure higher than enclosure 63. Between member 17 and the work W there is a channel 75 through which a shielding gas, typically helium or other gas is supplied, to assure, along the beam path, the presence of a gas, replacing air, which gas minimizes electron beam scattering and also to provide inert shielding at the work.

The rod 31, hole 77 in anode A, and holes 51, 53, 61, 71 are coaxial and aligned. The members 11, 13, 15, and 17 are composed of thermally and electrically conductive material and are grounded. Beam current impinge on the aperture walls flows to ground.

The Electron Beam Generator includes a coil 81 for producing a magnetic field focusing the beam electromagneti- :ally and coils 83 for producing a magnetic field to deflect the beam. The coil 81 is supplied with direct current to produce an adequate field for focusing The coil 83 may be supplied with varying direct current to center the beam in the holes 61 and 71. The centering in holes 51 and 53 may be effected by :ilting the anode A. The beam is focused electrostatically by :ooperation of the fields produced by the control electrode G and anode A and electromagnetically by the field of coil 81. As shown in FIG. 3 each field produces a crossover region, 85 or the electrostatic field and 87 for the magnetic field. The livergence of the beam is shown highly exaggerated in FIG. 3.

\ctually the half-angle a of the beam does not exceed Vz to 1 n each region where it diverges or converges.

It is desirable that the impingement of the beam on the aperure walls shall be minimized. This is most effectively achieved y setting the electrostatic focus so that the crossover region :5 is approximately midway axially between members 11 and 3. The electromagnetic focus may then be set so that the rossover region 87 is at, just outside, or just inside the work. or any setting the shifting of crossover region also causes rossover region 87 to shift. This invention arises from the realization that it is such shifting which caused the damage to apertures experienced in the prior art practice. In addition the shifting results in undesired or improper impingement of the beam B in the work.

The power supply includes an anode power source 101 for impressing the necessary high anode-cathode potential for concentrating the electrons from electron source 31 into a beam B. This source 101 includes a variable autotransformer 103, such as a VARIAC transformer, whose primary 105 is supplied from the buses or lines L1 and L2 of an alternating current commercial supply. The adjustable secondary 107 of transformer 103 supplies the primary 109 of a high-voltage step-up transformer, whose secondary 111 in turn supplies a high-voltage rectifier bridge 113. The positive DC terminal of the bridge 113 is connected to ground through a milliameter M; the negative to the rod 31 through a resistor 115, typically of several thousand ohms. The resistor 115 is shown variable; it may consist of a fixed component say of about 2,000 ohms and a variable component say of 3,500 ohms. The resistor 115 may be of the nonlinear or nonohmic type having a characteristic adapted to the desired compensation of focal shift.

The Power Supply also includes a filament power source 121 of substantially lower voltage and internal impedance than the source 101 for energizing the filament 33. This source 121 also includes a rectifier 123 energized from lines L1 and L2 through a variable transformer 125 and an isolating transformer 127. The positive terminal of rectifier 123 is connected to conductor 27, and the negative terminal to conductor 25.

The Power Supply also includes a source 131 for accelerating the electrons from filament 33 which bombard the rod 31. This source also includes a rectifier 133 energized from conductors L1 and L2 through a variable transformer 135 and an isolating transformer 137. The negative terminal of rectifier 133 is connected to conductor 25 and the positive terminal to conductor 23.

The variable transformers 103, 125 and 135 serve to impress variable or adjustable parameters on the beam. Transformer 103 varies the voltage impressed between anode A and cathode K; transformer 125 varies the heating of filament 33 and transformer 135 the bombardment of the rod 31 by electrons and the heating of rod 31. Variable transformers 125 and 135 thus vary the number of electrons produced at the rod or the electron current of the beam. It has been discovered that as these parameters are varied the focusing of the beam shifts spacially and that this shifting causes damage to the members 11, 13, 15, 17 because of the impingement of the beam on the walls thereof. The impingement occurs first at the narrow ends of the members and the damage has been ob served near these narrow ends.

There is also a bias source 141. This source also includes a rectifier 143 energized from lines L1 and L2 through a variable transformer 145 and an isolating transformer 147. The negative terminal of the rectifier 143 is directly connected to the control electrode G; the positive terminal to the adjustable tap 149 of the resistor 115. The source 141 and the resistor 115 together impress a negative bias on the electrode G which has a fixed component and a variable component dependent on the beam current. This variable component introduces an instantaneous compensating field into the electrostatic lens composed cooperatively of the fields of electrodes G and A. The compensating field maintains the focus of the beam B, so that the crossover region does not depart appreciably from the position shown in FIG. 3 as the parameters in the beam 8 are varied. It has been found that the source 141 may be entirely dispensed with and reliance placed alone on the resistor 115 for bias. Where the source 141 is present it provides another facility for changing the parameters on the beam B. The data presented below was developed with apparatus which did not include a source 141.

The practice of this invention has been subject to investigation with Electron Beam Generators having different anodecontrol-electrode cathode dimensions. Four different sets of dimensions, I. II, III, IV, are shown with reference to FIG. 2.

in the following table A manually or by mechanicalcoupling to the mechanisms which change the parameters. But important to realize that un- Generators in which the structures are as shown in rows II, III and IV of Table A have been used successfully in the practice of this invention.

The shifting of the focal regions of the beam B as the parameters in the beam are varied is illustrated with reference to FIGS. 4 and S. In FIG. 4 the computed equipotential lines in the region between the control electrode G and the anode A are shown. Axial distance from the end of rod 31 in inches, along the Generator column is plotted horizontally and radial distance in inches vertically. In FIG. 5 the computed beam trajectories for the equipotential lines shown in FIG. 4 for different electron energies are presented. Axial distance in inches from the cathode K is plotted horizontally and the radius of the beam in mils (0.001 inch) horizontally.

As shown in FIG. 5 the minimum beam diameter occurs between 2 inches and 3 inches from the cathode K; this region is between the cathode K and the anode A. At higher beam currents, because of space charge flattening of the equipotential lines near the emitting surface of the bolt 31 depending on the cathode configuration, the focal point tends to shift away from the cathode through members 11 and 13 which are approximately 5 inches from cathode K in typical electron beam apparatus, and eventually beyond them. In the absence of the refocusing effected in the practice of this invention, so that the crossover occurs and remains between the members 11 and 13, excessive energy is intercepted by the orifices and they are damaged.

The evaluation of this invention was carried out with the structures shown in table A and FIG. 2. Configuration l cathode which is used without grid bias tends to be too low in perveance, i.e., at 150 kV it is not possible to obtain the desired loadings of 80 to I ma. For the higher perveance cathode structures (see configurations II, III and IV of Table A) it was determined that the tendency of the crossover point of the electrostatic focus to shift beyond the desired inches cathode-toorifice distance could be compensated by allowing the grid bias potential to become more negative relative to the bolt as beam current increases. Perveance is essentially the value K in the equation I=KV3/2 where [is the electron beam current and V the voltage.

The equipotential plot, FIG. 4, is based on the use of-l50- volts grid bias. Increasing bias increases the curvature of the equipotential lines. increasing the inward radial component of electric field. and thus brings beam focus back to the region between the members 11 and 13 where it is initially set between these members. In one of its aspects this invention arises from the discovery that the use of increasing negative grid bias corrects the tendency of beam focus to shift with increasing space charge to points beyond the region between members 11 and 13 for the higher current cathode structures II, III, IV.

The compensating variation of the grid bias to compensate for the shifting of the focal position could be effected by changing the setting of an adequate variable bias source either less the compensating setting becomes effective substantially instantaneously the apertures are damaged before the setting takes effect. This property of the apparatus militates against manual setting and often against coupled setting. In addition, bearing in mind that the source 141 may be dispensed with altogether, an adequate variable bias source is costly. It adds weight and size to the Power Supply and where the setting is manual, represents an additional variable which must be controlled by the equipment operator as he carries on with the already complex task of adjusting all of the variables associated with the apparatus. Furthermore, since the distance from the crossover of the electrostatic lens to the magnetic lens is the object distance of the magnetic lens, excursions of beam focus of the electrostatic lens with changes in beam current must be corrected by changing the focal length of the magnetic lens, an additional complication.

It has been discovered that it is possible to use the bias resistor 115 in series with the cathode K (in this case the electron-beam-heated bolt 31) to develop the necessary increasing grid bias potential as beam current is increased. It has been found experimentally possible to determine values of this bias resistance which permit holding the electrostatic focus reasonably constant as beam current and voltage are varied. Thus a 7,000-ohm cathode bias resistor 115 has been found to be an effective source of grid bias for configuration II of table A and a 5,000 to 6,000-ohm resistance 115 has been suitable for configuration III oftable A.

As the beam current increases and the increase in space charge tends to throw the beam crossover axially away from the cathode K, the increased grid bias potential developed across the cathode resistor 115 tends to bring it back. For the range of currents required in the apparatus when used for welding it has proved possible to develop a configuration of the cathode and grid assembly which together with a resistor of proper magnitude gives reasonable transmission of the beam through the orifice system without overheating the orifice members as the beam power is adjusted from 2 kw. at kv. to 12 kw. at 150 kV. The use of the cathode bias resistor 115 also has a stabilizing effect on beam current. For example, if the supply voltage (Ll-L2) were to increase and thus tend to draw a higher beam current, the higher grid bias potential produced at the higher current would tend to reduce the value of current which would otherwise flow.

This invention in one of its aspects contemplates an electrostatic telefocus electron beam focusing system and a bias resistor 115 in series with the beam current at the cathode K for the purpose of developing a value of negative grid bias potential which increases linearly with beam current and thus tends to maintain the focused condition. As indicated above, such a system eliminates the need of a separate gold bias supply, is more readily controlled, and is less costly, lighter and smaller than a system having a separate grid bias supply.

The following table B presents data showing the effectiveness of the control achieved in accordance with this invention:

6 After5m1n. changing 1.1 tim TABLE B Bias Kv. (avg. corr.)

Cathode power, in

w. K9 Remarks pm. pm, nn!" Ll p. KV-(z) ma. ma. ma. ma. percent Unsmblv.

Much more smbhn Unstable-L increasing.

v u u 5 97 2.4 IZLrJ L75 7 1 J 2 0 6. 4% 04 7% 344M667M8M113MUM6 33456 1 'IABLEB Kv. Cathode Bigas pm, pm, pTotn I, I/I;,, (avg. power, 1n Kv.m ma. ma. ma. ma. percent 0011.) W. KO Remarks KVA (1) TABLE C Deflection coils Filament Cathode Focus D! Dy IL IF VF In V13 (ma.) (ma.) (amp) (amp) (volt) (ma.) (volt) Remarks Kv. Percent (volt) It (ma.)

I (ma) C w CCC on o o 0 821 h 34 5 m 51H 6902350 358023 9 37 3 0011111 llifiwmmwmnnu m mm 0 m wZ/m 1% :D-D-DKU-5555555555 mmwmm LLL1 1 1 mwmmmm m 511222 011111 a 111111 589524 51 5 57 m su. .awmmnmmmmm w.

0 0 0 D 5 0005 000000 25 601701 6 l 11 234411 lmqufiwwmuumww mu voltage in volts across ii l bombardment current in milliamperes to bolt 31 ln table C the columns have the following significance:

total anode-cathode current in milliamperes V bombardment voltage (131) in volts.

= current transmitted through apertures in milliamperes kV anode-cathode potential in kilovolts Table D below is a table similar to Table C taken under the same pressure conditions for a Generator of Confi but with a 6,000-ohm resistor 115:

percent:

primary voltage in volts of high-voltage transformer D current through deflection coils 83 in milliamperes 30 guration II] current in amperes through focusing coil 81 Table E below is table taken under the same conditions as C t .m n 8W 6 m D. ho .m fim m m. d .1 t am M fi 0 n Pm Mu 0 m B 41H 48 2 47 13 5 1 5 m MQUMUWQQMOU "%9%m0%0 U u 0 11 111. 111.1 111 v n N BJ 50.00%22M500 0550655 5558255 67 5 67 44-055 m m %22% M2 22% W%22222% m u a u U u d h M 766666666 6 6666666 66666666 I. V 9 0 0 0 0 0 0 0 0 .0 0 0 0 flw0 0 0 N 0 0 0 0 fiw0 00 v 11111111 -1 -llllll 1 11111111 m w n u .S h n u 455556455 6 6521 0 5 95 m up a sin m H 55 mam m me m 6656 5 5 5 5 5 5 44 n m H w u m y) p D8 0 D m n a E D L X). B Da 0 A m n T a m a 0 m v m w M 5070055612805367500 0526425 2 D 6 2 9 -D .D fiw7 8 7 &7 4 fiw7 9 1 0 6 7 m m 888888888888888889 H wweo owm omeo 6 m h m a old 555090555586500060000002 505 a .6 5 5 1 om $0 5 .3.7.8.7.7.5.7..D.&9. 4 h m 81234 m23M55 1234 566m nln m w m g W r h I S B r e p n I a n t n C r r u C NOTE.-When H.V. off, IF nndIn increase-possibly X-ray efiect on photoconductors.

TABLE E l}. 1. (mm) (mm) Percent Kv.

.120503671592 5 A3310-233loa223%3m% u .OOOGFOOW-DESOOKUEOB Table G below is a table similar to table F, taken under the same conditions, but with the resistor 115 at 2,500 ohms.

Tables H and .l are like tables, taken under the same conditions as C through G, for a Generator of Configuration 1V and with resistor 1 15, 6,000 and 4,000 ohms, respectively.

In Tables H and .l the column headed 13(C.) presents the temperature of apertures 13 which was water-cooled.

Table K is a like table for a Generator of Configuration 1V and with resistor 115, 5,000 ohms. Helium was supplied at slightly above atmospheric pressure at 120 C.F.H. through the channel 75 for the rows labeled He ON."

TABLE F Filament V I], 1? VP (volts (amp) (amp) (volt) 1 It (may (ma.) Percent XV.

ln this case the Generator was of Configuration ll and the resistor 115, 6,000 ohms. The expression S.C.L. in the left-hand colunm means space-charge limited at the voltage impressed; that is, the beam current was the maximum which could be derived from the bolt 31 at the anode-cathode voltage (101) impressed.

The column headed 11 Temp C." gives the temperature in Centigrade degrees for aperture 11 and the column, Temp C. for aperture 15.

Table F below is a table similar to Table E, taken under the same conditions, for a Generator of Configuration II and resistor 115, 5,000 ohms.

72 6 1 1790 6502 A 3) 4. 1 2 w a 1% 225555344 m m m m m m um mmmmmwmmunmwmmmmmuma m u c e u t n n u n n n 778838877 878 871 1 7 111111111 wm c lllwwll2m 1W nwmmu mwm www mm n u u -W.%%M%M %%RMHWMMW o n n H u o .5 o n m m m m m 1. 902 02369370 5 2 9 2 7 5 5555 5) 55555505 1 122555111212 1 mmmwowmawnamaommnfimoz mmmmmmmwm 15 mnwwwmmwm 5 o A o t m 1 0 16 15002 1 9 79 82 8 281 0 o B B) 3811111 3 H 9mmmun %m9%% m WWMMMMN9WMMWO0N9OMW 112Mm%%%%3 t 1122222%2 0 0 1111 1111111 11 111111111 Vd 111111111 W V V l. 1 O 0 B 05 5008 5 505 B 2082 B 5 55 5% 05 GWOO 5.39 5 9 55 5 39 55 9 9 15 oxmmmaammmxwm 15 wmwm m momma wmaawm 6 6 6 666666 1 M 6 6 6666666 m n n 5 M m H H F 5555555555555 F 9990999999999993333 54% %1 a hm 4 2221O m V m Q 9 00 9 9 9 9 9 D 9 9 h V 8 &8 nm&&&&&&&&om&&8 &&8 oo 1 5 .55544 m .55 05 0 V /V\ G t 6 F 55 .D 2 .J t 5 H E m Fm oszwlw bmw E m no ufiwawowooamummxmw w mwwwunwnomlx L 5) 007m%m4m1 m m 5 5 5 5 5 5 5 L m m .m 6 6 &5 5 5 5 5 5 &144 H 8 L m m ..n..n.. .w r.nm m m m 111 11 111 F T m T L 0 5 H 1m mambo mmmmoo N no mmmwnmunmmwmwomaoooo 1 1 .L1 1 1 11 L1 m LLLLLLLLLLLLLLLLLLLL 21445555. v) 1234 4 454 E H 1 a wnnnmmnww N vm wnnummnmw L u u M w m h n n 446 0 89 55 o 0 5 o 000 T m mfimm 535mm 0 wmmmumamomwwoonomm V mmmm m 0 11111 111111 m 11111111111111.111111 111111111 K 111111111 V V 3 9665650 t 938551522 0 u 0 0 9% n 89 9 9 v mmmmwm %%%wm v mwwwww%%%%%%%%%5wm.w6 9 9 m K 11111 11111 n K 11111111111111111111 e W t 5 3 7 80 P n W8R -M mm m m .HJWMMJMMJAJJ w w mfi u w e 0 g 9 1 9 5 e 2 3 582 94052 2 1511504 C 8 7 no au 5 C 8 no 777788 66 %345 .6 .33 .44 M u n A H 12 5 t m m Mm P u n P 575555002975 5 0652150510 455 I 012 .1 I 123 .5 .7861 222938456 25555 885 B t B 29 .m334 a M m 8 WWW H H" m N QM w 6 2 l 5 2 3 no !I\ wk m u TWA M M 0 u u w cu 1 LLLLLL LLLLLL m a u m m n o o ooo m m n n U SSSSSS SSSSSS u u u Tw( w WNW u n u n u n n n n n d n "5 m h 5 0 .5 u 0 n0 C 15 SH Mm H TABLE K Bolt n (mm) Filament w I (amp) (amp) (volt) V1; 16 l 13 11 '(volt) 1 c.) (90.) 0.)

In I; Per- V (1118.) (mm) cent Kv. '(vollj 50 0 9 wimiiwiwmm mmamnfifi 5 .n5 5 5 5 5 4 5 5 4 LILILLILLLLLLLLLLLLLL 0 1 1 1 1 0 D Q0 1 1 123466612MM- He ON Tables L, M, N,O, P are like tablesfor aGenerator of Configuration Ill and with resistor of 5,000, 2,500, 6,000, 7,000, and 5,000 ohms, respectively.

TABLE L 15 13 11 (1118) (volt) C.) 0,) C.)

Bolt

Filament ID 10, VD (11121.) (mm) Percent Kv. (volt) (amp) (am p) (volt) It panel meter 5056 02 2 4322 24 %%4 5 5 fiw5 -D 5 5 5 5 5 TABLE M LLLLLLLLLLLL Worse than kv Filament Bolt IF VF I13 VB (amp) (amp) (volt) (11121.) (volt) 0.)

VD (Volt) 19 I: (ma.) (ma..) Percent Kv.

566 6 0-D lllmlmnnllm 0000 09 ml234n xmo2w 11111111 48 65 m 0000055000 mmllz 31m Tripped TKFLE Bolt Filament (mm) (volt) C.)

IF VF p) (volt) VD IL (volt) (amp) I,1 It (11111.) (MA) Percent Kv.

94 67 4.0 2 17 561 llllmllwm2 2 2lmmll2 02 55 8 5 5556550 5 050 o wmmnwwmwmwmwmmw mmmnwnmwmw 222222222227777777777777777 9 90mownwumflmflmflwnmnmom-hemnmnmoaomomomumomomomomomum TABLE N Filament Bolt I; V I IF V I Va 15 13 (ma) (MA) Percent Kv. (volt? (amp) (amp) (volt) (ma) (volt) C.) 0.) C

52 4s 92. 3 7 4 36 71. 5 64. 5 9 7 25 47 33 82. 74.0 90 2 7 25 40 43 45CFII1IeON 83 72 115 CFiI/He ON--." 84 74 88.2 50 38 46 10 8. 3 83 100 104 1. 20 7 265 90 14 12 18 20. 19 92. 8 100 114 1. 20 8. 7 290 94 15 12 17 34. 5 33 97. 1 100 114 1. 17 8. 7 305 99 17 12 12 50 48 95. 9 100 116 1. 15 8. 7 335 101 23 10 11 11c ON-120 CFI{ 51 46.5 91.2 100 40 10 11 54. 5 52 95. 4 110 128 1. 23 4. 65 8. 7 335 101 23 12 12 57.5 54. 5 94.8 120 137 1. 27 4. 65 8. 7 335 101 23 17 14 61 57. 5 94. 3 130 147 1. 31 4. 55 8. 7 335 101 23 25 17 73 93. 1 140 157 1. 33 4. 45 8. 7 335 103 25 33 24 81 74. 2 91. 5 150 168 1. 38 4. 4 8. 7 325 103 25 47 30 TABLE 0 Filament Bolt Orifice '1 I I. V., 11. IF V1 IB VB 15 13 (ma) (ma) Percent Kv. (volt) (amp) (amp) (volt) (ma) (volt) 0.) C.) 11 C.)

10. 0 7. 9 79 100 5. 25 9. 6 250 89 17 14 23 20. 0 17. 5 B7. 5 100 5. 1 9. 6 264 93 21 24 20. 0 19. 0 95. 0 100 .20 5. 0 9. 6 260 92 22 15 15 30. 0 28. 2 94.0 100 116 1. 18 4. 95 9. 6 280 94 22 15 15 40. 0 37.0 92. 5 100 116 1. 17 4. 9 9. 6 295 97 25 20 17 50. 0 47. 5 95. 0 100 117 1. 15 4. 9 9. 6 310 99 27 15 16 S.C.L- 57.5 55. 0 95. 7 100 117 1.17 4. 9 9. 6 345 104 30 13 15 10. 0 9. 5 95. 0 125 125 1. 36 5. 15 9. 6 240 87 15 13 22 20. 0 19.0 95. 0 125 138 1. 4. 95 9. 6 255 92 18 13 21 30. 0 29. 0 96. 8 125 140 1. 34 4. 9 9. 6 270 93 22 15 15 40. 0 37. 0 92. 5 125 141 1.32 4.85 9. 6 285 96 22 32 20 50.0 45. 5 91.0 125 142 1.30 4. 85 0. 6 300 98 22 36 31 60. 0 55. 5 92. 5 125 142 1. 29 4. 85 9. 6 310 100 23 36 32 70. 0 66. 0 94. 3 125 144 1. 29 4. 80 9. 6 320 102 27 25 23 S.C .L 76. 5 74. 0 96. 9 125 144 1. 30 3. 80 9. 6 365 108 30 13 14 10. 0 9. 0 90. 0 150 146 1. 49 5. 2 9. 6 245 87 13 13 22 20. 0 19. 0 95. 0 150 155 1. 48 5. 0 9. 6 260 91 17 13 27 30. 0 29. 0 96. 7 150 163 1. 46 4. 8 9. 6 260 93 22 18 40. 0 37. 0 92. 5 150 165 l. 4. 75 9. 6 270 95 20 36 20 51. 0 44. 0 86. 3 150 166 1. 44 4. 70 9. 6 280 97 19 46 70. 0 60. 0 85. 7 20 Tripped TABLE P Filament 5.5 init. Bolt Orifice T I I. V 11. I1 VF I11 VB 15 13 11 (ms..) (ma.) Percent Kv. (volt) (amp) (amp) (volt) (ma) (volt) 0.) 0.) C.)

10.0 9. 0 90 150 146 1. 51 5. 0 9. 5 250 88 16 16 36 20.0 18.0 90 150 154 1. 50 4. 85 9. 5 260 92 17 17 45 30. 0 27. 5 91 8 150 164 1. 48 4.65 9. 5 260 94 24 18 43 40.0 38. 5 96 4 150 164 1. 46 4. 9. 5 270 96 25 16 33 50.0 49. 5 99 0 150 165 1. 45 4. 55 9. 5 275 97 23 21 23 60. 0 60. 0 100 150 166 1. 45 4. 55 9. 5 282 99 2A 16 70. 0 70. 0 100 150 167 1. 43 4. 5 9. 5 290 100 23 25 16 80. 0 79. 0 98 7 150 168 1. 42 4. 5 9. 5 300 102 24 26 18 120 CF11 Ho ON 80.0 76.0 52 26 8 10. 0 9. 5 9. 5 220 88 14 13 21 20. 0 19. 5 9. 5 205 93 18 13 22 30. 0 30. 0 9. 5 280 96 18 12 15 40. 0 40. 0 50. 0 50. 0 9. 5 305 100 21 12 14 8.0 .L 55. 0 55. 0 9. 5 315 102 1 1 16 64. 0 65. 0 9. 5 305 102 23 13 14 5.0 .L 74. 5 74. 0 9. 5 320 103 33 13 15 21 10.0 10.6 250 97 23 17 58 30 15. 0 100 Table 0 below 15 a llke table produced with a Generator of Configuration III and wlth a resistor 115 of 5,000 ohms:

TABLE Q Filament Bolt Ip 11 L 1 VF In V3 15 13 11 (mo (ma) Percent Kv. (amp) (amp) (volt) (ma) (volt) 0.) 0.) 0.)

l0 5. 1 51 100 1. 20 5. 35 10. 3 215 93 14 15 46 21 12 100 100 1. 20 5. 35 10.3 230 97 18 18 68 10 7. 1 71 100 1. 22 4. 8. 2 245 99 17 16 32 20.5 16. 5 80 5 1. 23 4. 75 8. 2 260 103 20 16 35 3O 25. 8 100 1. 22 4. 7 8. 2 280 105 20 16 36 40. 5 37 91 3 100 1. 20 4. 7 8. 2 295 107 25 15 24 46 42. 5 92. 5 100 1. 19 4. 65 8. 2 305 108 30 14 16 50. 5 47. 8 96. 5 110 1. 26 4. 65 8. 2 305 108 27 13 15 60.5 57. 5 95 1.30 4. 6 8.2 310 110 32 14 15 65 62. 3 96 131 1. 37 4. 55 8. 2 310 110 28 13 15 70. 5 66 93. 7 4. 55 8. 2 310 110 40 14 16 76. 5 73 95.5 1. 42 4. 45 8. 2 305 110 35 13 15 82. 5 79 95. 7 1. 45 4. 4 8. 2 305 110 35 13 15 12 mm He ON 10.0 6- 3 63 100 21 20 14 2 30.5 22 72 100 1. 18 32 15 23 41 32 78 100 1. 17 35 14 2 45. 5 37 81.3 115 1. 26 36 14 20 50. 3 4a. 5 so 130 1. 35 37 14 21 2 Beginning with the 5th row from the end the collector 200 was replaced by an electrically insulated water-cooled copper block (not shown) connected to ground through a meter M2. Except where I20 cubic-feet-per hour helium is indicated on vary the beam current but to maintain the electrostatic focus.

This invention has been disclosed herein to be practiced with electrons. To the extent that this invention may be practiced with charged particles of other types, such practice the left, the channel was sealed off as for previous tables. 5 id i hi h Scope h f The fohewlhg table R Presents like data with e Same P While a preferred embodiment of this invention has been p f as for e last five rows Q h with ah disclosed herein, many modifications thereofare feasible. This mlhum block without water Cooling replacing the Copper invention is to be restricted only to the extent necessitated by block- 10 the spirit of the prior an.

AB E R We claim: 7 Per- IL 15 0 C13 0 11 l. The method of treating a workpiece with electrons which cent (amp) C comprises the steps of generating a focused beam of electrons 2 S8 37 58 1g 29 of a current magnitude such that the impingement thereof on 32: 130 ""i 3i6' 32 1 20 the workpiece will not injure the workpiece, of relatively moving said beam of electrons and said workpiece so that said The following table S presents like data taken with the apbeam of electrons impinges on said workpiece solely at the loparatus used for table C but with the gasket 202 absent and cation that is to be treated, and thereafter increasing the curthe collector 200 spaced about one-fourth inch below the gas 20 rent ofsaid beam ofelectrons and substantially simultaneously channel 75, and with He at about 120 C.F.H., slightly above refocusing said beam to compensate for any defocusing atmospheric pressure, flowing through the channel. caused by the increase in current.

TABLE S 6.2 amps initial I I; I IF VF I1; v5 15 13 11 (1118,.) (ma.) Percent Kv (amp) (amp) (volt) (ma.) (volt) C.) 0.) C.) 7 70 100 1. 24 6.6 8.3 270 105 17 13 2s 20. 5 15 73. 2 100 1. 5. 5 8. 3 290 107 24 15 37 44. 5 37 83. 2 100 1. 20 5. 4 8. 3 315 111 46 15 26 S.C.L 5O 42 84. 0 100 1. 20 5. 4 8. 3 320 111 15 21 I0 8 125 1. 34 5.4 8. 3 265 96 15 12 21 31 26. 5 85. 5 125 1. 35 5. 2 8. 3 285 102 21 14 28 50. 5 45. 5 125 1. 36 5. 05 8. 3 295 105 36 14 14 60 54. 5 90. 8 125 1. 36 5. 05 8. 3 295 105 41 16 14 10 7. 85 78. 5 150 1. 49 5. 2 8. 3 245 94 15 12 24 31 26 84 150 1. 49 4. 8 8. 3 250 25 16 43 52 45. 5 87. 5 150 1. 49 4. 8 8. 3 255 103 35 17 39 72 65 90. 3 160 1. 49 4. 65 8. 3 260 105 46 15 29 81 78 90. 2 150 1. 49 4. 65 B. 3 265 106 58 16 27 Table T presents data with the apparatus used for welding and with the work W (stainless steel), AlSl 304) spaced onehalf inch below gas channel 75, and with He gas at C.F.H. and slightly above atmospheric pressure flowing through channel 75. Each row corresponds to a different weld. The Generator was moving relative to work to produce a bead on plate. in this case the Generator was of Configuration Ill and the resistor 115, 5,000 ohms. Loss from electron scattering by the backflow of helium through the members 11, 13, 15, 17 and loss from reflection of the beam at the work W are the major reasons for the difference between 1,, and I,.

TABLE T n t IL 15 13 11 (11121.) (ma) Percent Kv (amp) C.) 0.) C.)

Table T corresponds to operation of the apparatus at its maximum continuous rated power output. The apparatus was operated for welding over the current range of 1,, down to 20 milliamps and proved satisfactory as is to be anticipated from the above tables.

The data in tables C through T shows that the focusing may be set for high beam currents (50 milliamps or above) so that the current transmission of the beam through the orifice members exceeds 90 percent and that the shift of the focusing as the beam current changes may be suppressed or minimized so that at very low currents l0 milliamperes) the current transmission of the beam is about 70 percent.

The apparatus according to this invention is operated primarily substantially below the space-current limit (SCL) and the beam current is varied predominately by varying the :ransformers and that is, the filament-heating current and/or the bombardment voltage. The grid G serves not to 2. The method of claim 1 in which said step of generating said beam of electrons is in a first area maintained at a pres- 0 sure which does not exceed a few microns, said step of moving said workpiece is in a second area of substantially greater pressure and of passing said beam from said first to said second area through at least one aperture.

3. The method of claim 2 in which said first area is maintained at a pressure not in excess of V2 micron and said second area is substantially at atmospheric pressure.

4. The method of treating a workpiece with a beam of electrons with apparatus including a source of electrons and means for varying the electrical parameters controlling the electrons from said source from a first setting in which said beam is of low intensity which low intensity is insufficient to affect said workpiece to a second setting in which said beam if of a high intensity which high intensity is sufficient to treat said workpiece, which said method comprising concentrating said electrons from said source into an electron beam, transmitting said beam into treating relationship with said workpiece through aperture means between said source and said workpiece, said aperture means including at least one aperture member having a wall defining a hole through which said beam passes, setting said parameter-varying means at said predetermined first setting, focusing said beam at said first setting in a position to preclude damage to said aperture means by impingement of said beam on said walls of said aperture member, adjusting the relative position of said beam and said workpiece so that said beam impinges solely on a desired portion of said workpiece, setting said parameter-varying means at said second setting and refocusing said beam at said second setting such as to preclude damage to said aperture means, said refocusing being effected in so short a time that said aperture means is not damaged before said refocusing is completed.

5. The method of claim 4 wherein the refocusing of the beam is effected substantially instantaneously responsive to the change from the first setting to the second setting.

6. The method of claim 1 wherein the step of refocusing occurs in response to the change in power from its said first magnitude to its said second magnitude.

7. The method of treating a workpiece with a beam of electrons with an apparatus which includes a source of electrons and means for varying the electrical parameters controlling the electrons from said source, the said method comprising concentrating said electrons from said source into an electron beam, transmitting said beam through aperture means from said source to said workpiece, said aperture means including aperture members having walls defining holes through which said beam passes, setting said parameter-varying means at a first setting to provide the current magnitude of said beam at a first value, said first value being sufficiently low so that said beam will not injure said aperture members and said workpiece, electrostatically focusing said beam at said first setting so as to minimize the direct diversion from said work by the walls of said aperture members of the beam current thereby to preclude later damage to said aperture members by impingement of said beam on the walls of said aperture members when said parameter-varying means is at a second setting to provide the current magnitude of said beam at a second value, said second value being sufficient to impart the desired treatment to said workpiece, relatively moving said beam and said workpiece so that said beam impinges solely on a portion of said workpiece which it is desired to treat, and setting said parameter-varying means at said second setting, the resetting of said parameter-varying means causing said beam to maintain the diversion from said work by said walls of beam current minimized thereby to preclude damage to said aperture members, said refocusing being effected in so short a time interval that said aperture means is not damaged before said refocusing is completed.

8. Electron-beam treating apparatus including a cathode, a source of electrons for said cathode, means concentrating said electrons into a beam, a control electrode for electrostatically focusing said beam as a function ofa bias potential established between said cathode and said control electrode, adjustable means for setting the electron current of said beam at different magnitudes selectable at the will of an operator, aperture means interposed between said source and said work and through which said beam passes, and means coupled to said control electrode and responsive to the magnitude of said electron current of said beam for substantially instantaneously resetting said bias potential and thereby the focusing of said beam on occurrence of a change in said parameter of said beam, the said resetting means including impedance means connected between said source and said cathode and connected between said electrode-and said cathode to impress a bias potential on said electrode, said bias potential having a magnitude which is dependent on the beam current and of a negative polarity and effective to maintain the desired focus of said beam.

9. The combination of claim 8 in which said resetting means alters said bias potential as a function of the alteration of the potential across said impedance means.

10. The combination of claim 9 in which said impedance means is a resistor connected in series with said source of elecw v i l n trons.

ll. Electron-beam apparatus for treating a workpiece comprising an apertured anode, an electron-emissive cathode, a control electrode located intermediate said anode and said cathode, a housing enclosing said anode and said cathode and said electrode, energy supply means for establishing a potential between said cathode and said anode, means establishing a low pressure in said housing whereby the establishment of said potential will enable a stream of electrons to flow from said cathode past said control electrode and beyond said anode to impinge on said workpiece, focusing means including said control electrode for controlling the area of the impingement of said beam on said workpiece, and in biasing means responsive to the power in said beam and operable to provide a bias potential which will maintain said control electrode at a negat1ve potential with respect to said cathode, variations in the magnitude of said bias potential being operable to effect a change in the operation of said focusing means whereby the focus of said beam is substantially uninfluenced by changes of the power in said beam.

12. The combination of claim 11 in which said focusing means includes an impedance traversed by at least a portion of the current flowing through said cathode.

13. The combination of claim 11 in which an impedance is connected in series between said energy supply means and said cathode and in which said focusing means establishes said control potential as a function of the potential established by the power flow through said impedance.

14. The combination of claim 13 in which said cathode is a cylindrical member having an outer diameter Db said anode is a platelike member with its said aperture substantially centrally thereof and of a diameter Da, said control electrode is a hollow cylindrical member through which said beam passes and having first and second apertured walls extending across the path of said beam, said first wall having a first surface which is nearest said cathode and spaced a distance along the path of said beam a distance Sgb and a second surface spaced from said first surface by a distance Sg, said second wall having a first surface which is furthest from said first wall and spaced a distance Sge therefrom, said anode having its surface nearest to said cathode spaced therefrom by a distance Sba, said aperture of said first wall of said electrode having a diameter Dg and said aperture of said second wall of said electrode having a diameter Dge, said focusing means including a bias potential source connected in between said cathode and said control electrode in series with at least a portion of said impedance.

15. The combination of claim 14 in which said impedance is a resistor, said resistor has a resistance of about 7,000 ohms, Db is 0.062 inches, Da is 0.312 inches, Sgb is 0.001 inches, Sg is 0.101 inches, Sge is 0.578 inches, Sba is L835 inches, Dg is 0.250 inches, and Dge is 1.180 inches.

16. The combination of claim 14 in which said impedance is a resistor, said resistor has a resistance of about 5,000 to 6,000 ohms, Db is 0.062 inches, Da is 0.312 inches, Sgb is 0,001 inches, Sg is 0.060 inches, Sge is 0.398 inches, Sba is L730 inches, Dg is 0.250 inches and Dge is 0.706 inches. 

1. The method of treating a workpiece with electrons which comprises the steps of generating a focused beam of electrons of a current magnitude such that the impingement thereof on the workpiece will not injure the workpiece, of relatively moving said beam of electrons and said workpiece so that said beam of electrons impinges on said workpiece solely at the location that is to be treated, and thereafter increasing the current of said beam of electrons and substantially simultaneously refocusing said beam to compensate for any defocusing caused by the increase in current.
 2. The method of claim 1 in which said step of generating said beam of electrons is in a first area maintained at a pressure which does not exceed a few microns, said step of moving said workpiece is in a second area of substantially greater pressure and of passing said beam from said first to said second area through at least one aperture.
 3. The method of claim 2 in which said first area is maintained at a pressure not in excess of 1/2 micron and said second area is substantially at atmospheric pressure.
 4. The method of treating a workpiece with a beam of electrons with apparatus including a source of electrons and means for varying the electrical parameters controlling the electrons from said source from a first setting in which said beam is of low intensity which low intensity is insufficient to affect said workpiece to a second setting in which said beam if of a high intensity which high intensity is sufficient to treat said workpiece, which said method comprising concentrating said electrons from said source into an electron beam, transmitting said beam into treating relationship with said workpiece through aperture means between said source and said workpiece, said aperture means including at least one aperture member having a wall defining a hole through which said beam passes, setting said parameter-varying means at said predetermined first setting, focusing said beam at said first setting in a position to preclude damage to said aperture means by impingement of said beam on said walls of said aperture member, adjusting the relative position of said beam and said workpiece so that said beam Impinges solely on a desired portion of said workpiece, setting said parameter-varying means at said second setting and refocusing said beam at said second setting such as to preclude damage to said aperture means, said refocusing being effected in so short a time that said aperture means is not damaged before said refocusing is completed.
 5. The method of claim 4 wherein the refocusing of the beam is effected substantially instantaneously responsive to the change from the first setting to the second setting.
 6. The method of claim 1 wherein the step of refocusing occurs in response to the change in power from its said first magnitude to its said second magnitude.
 7. The method of treating a workpiece with a beam of electrons with an apparatus which includes a source of electrons and means for varying the electrical parameters controlling the electrons from said source, the said method comprising concentrating said electrons from said source into an electron beam, transmitting said beam through aperture means from said source to said workpiece, said aperture means including aperture members having walls defining holes through which said beam passes, setting said parameter-varying means at a first setting to provide the current magnitude of said beam at a first value, said first value being sufficiently low so that said beam will not injure said aperture members and said workpiece, electrostatically focusing said beam at said first setting so as to minimize the direct diversion from said work by the walls of said aperture members of the beam current thereby to preclude later damage to said aperture members by impingement of said beam on the walls of said aperture members when said parameter-varying means is at a second setting to provide the current magnitude of said beam at a second value, said second value being sufficient to impart the desired treatment to said workpiece, relatively moving said beam and said workpiece so that said beam impinges solely on a portion of said workpiece which it is desired to treat, and setting said parameter-varying means at said second setting, the resetting of said parameter-varying means causing said beam to maintain the diversion from said work by said walls of beam current minimized thereby to preclude damage to said aperture members, said refocusing being effected in so short a time interval that said aperture means is not damaged before said refocusing is completed.
 8. Electron-beam treating apparatus including a cathode, a source of electrons for said cathode, means concentrating said electrons into a beam, a control electrode for electrostatically focusing said beam as a function of a bias potential established between said cathode and said control electrode, adjustable means for setting the electron current of said beam at different magnitudes selectable at the will of an operator, aperture means interposed between said source and said work and through which said beam passes, and means coupled to said control electrode and responsive to the magnitude of said electron current of said beam for substantially instantaneously resetting said bias potential and thereby the focusing of said beam on occurrence of a change in said parameter of said beam, the said resetting means including impedance means connected between said source and said cathode and connected between said electrode and said cathode to impress a bias potential on said electrode, said bias potential having a magnitude which is dependent on the beam current and of a negative polarity and effective to maintain the desired focus of said beam.
 9. The combination of claim 8 in which said resetting means alters said bias potential as a function of the alteration of the potential across said impedance means.
 10. The combination of claim 9 in which said impedance means is a resistor connected in series with said source of electrons.
 11. Electron-beam apparatus for treating a workpiece comprising an apertured anode, an electron-emissive cathode, a Control electrode located intermediate said anode and said cathode, a housing enclosing said anode and said cathode and said electrode, energy supply means for establishing a potential between said cathode and said anode, means establishing a low pressure in said housing whereby the establishment of said potential will enable a stream of electrons to flow from said cathode past said control electrode and beyond said anode to impinge on said workpiece, focusing means including said control electrode for controlling the area of the impingement of said beam on said workpiece, and in biasing means responsive to the power in said beam and operable to provide a bias potential which will maintain said control electrode at a negative potential with respect to said cathode, variations in the magnitude of said bias potential being operable to effect a change in the operation of said focusing means whereby the focus of said beam is substantially uninfluenced by changes of the power in said beam.
 12. The combination of claim 11 in which said focusing means includes an impedance traversed by at least a portion of the current flowing through said cathode.
 13. The combination of claim 11 in which an impedance is connected in series between said energy supply means and said cathode and in which said focusing means establishes said control potential as a function of the potential established by the power flow through said impedance.
 14. The combination of claim 13 in which said cathode is a cylindrical member having an outer diameter Db said anode is a platelike member with its said aperture substantially centrally thereof and of a diameter Da, said control electrode is a hollow cylindrical member through which said beam passes and having first and second apertured walls extending across the path of said beam, said first wall having a first surface which is nearest said cathode and spaced a distance along the path of said beam a distance Sgb and a second surface spaced from said first surface by a distance Sg, said second wall having a first surface which is furthest from said first wall and spaced a distance Sge therefrom, said anode having its surface nearest to said cathode spaced therefrom by a distance Sba, said aperture of said first wall of said electrode having a diameter Dg and said aperture of said second wall of said electrode having a diameter Dge, said focusing means including a bias potential source connected in between said cathode and said control electrode in series with at least a portion of said impedance.
 15. The combination of claim 14 in which said impedance is a resistor, said resistor has a resistance of about 7,000 ohms, Db is 0.062 inches, Da is 0.312 inches, Sgb is 0.001 inches, Sg is 0.101 inches, Sge is 0.578 inches, Sba is 1.835 inches, Dg is 0.250 inches, and Dge is 1.180 inches.
 16. The combination of claim 14 in which said impedance is a resistor, said resistor has a resistance of about 5,000 to 6,000 ohms, Db is 0.062 inches, Da is 0.312 inches, Sgb is 0.001 inches, Sg is 0.060 inches, Sge is 0.398 inches, Sba is 1.730 inches, Dg is 0.250 inches and Dge is 0.706 inches. 